The present invention relates generally to a method and apparatus for efficiently cleaning a substrate processing chamber of residue, and more particularly to determining the endpoint of a plasma cleaning process.
One of the primary steps in the fabrication of modern semiconductor to devices is the formation of a layer, such as an oxide or nitride layer, on a substrate or wafer. As is well known, such layers can be deposited by chemical vapor deposition (CVD). In thermal CVD processes, reactant gases are supplied to the substrate surface where heat-induced chemical reactions take place to produce a desired film. In typical plasma-enhanced CVD (PECVD) processes, reactant gases are activated in a glow discharge plasma by the application of energy, such as radio frequency (RF) energy, to a reaction zone proximate to the substrate surface. This type of plasma is commonly called an in situ plasma.
During CVD processing, reactant gases are released inside the processing chamber to form layers, such as silicon oxides or nitrides, on the surface of the substrate being processed. However, during such deposition processes, deposition of undesired oxide or nitride residue can occur elsewhere in the CVD apparatus, such as on the walls and passageways of the processing chamber and on chamber components. Over time, failure to clean the residue from the CVD apparatus may result in degraded, unreliable processes and defective wafers.
Typically, two types of cleaning procedures have been used. "Dry cleaning" processes may be performed between deposition processing steps without opening the chamber. A dry clean may be performed after each wafer has been processed, or after n wafers have been processed. "Wet cleans" typically involve opening the processing chamber and physically wiping down the reactor, including the chamber walls, exhaust ports and other areas having residue, with a special cloth and cleaning fluids, and are normally done less frequently than dry cleans. Without these cleaning procedures, impurities from the residue built up in the CVD apparatus can migrate onto the wafer, or particles may fall onto the wafer.
In some instances, an in situ plasma may be used in a dry cleaning process. For example, PECVD systems typically include plasma systems that allow an in situ plasma clean. However, in situ plasma systems may be optimized for layer deposition, and not chamber cleaning. The efficiency of an in situ plasma cleaning process may also depend on plasma density and distribution, and may not adequately clean portions of the chamber not exposed to the in situ glow discharge. Therefore, an in situ plasma cleaning process may not be optimal to clean some substrate processing systems. However, other plasma cleaning processes may be attractive.
Remote plasma generating systems have been shown to be useful in cleaning substrate processing chambers. Remote plasma generating systems generate a plasma outside of the reaction zone in the processing chamber, and then flow plasma species into the chamber. The plasma species react inside the chamber, to remove unwanted residue, for example. Remote plasma generating systems can be added to substrate processing systems, with or without in situ plasma capability, in a variety of configurations.
Some remote plasma generating systems use a waveguide to convey microwave energy from a microwave source to an applicator tube where a plasma precursor gas is converted into a plasma. This plasma is conveyed down the applicator tube to the processing chamber where a chemical reaction between the plasma species and the residue occurs. Even though some recombination of plasma species occurs between the time the precursor gas is irradiated with microwave energy to form a plasma and the time the plasma interacts with the residue, using microwave energy to generate the plasma is quite efficient and often results in higher cleaning (etch) rates than an in situ plasma clean.
It is desirable to know when a dry clean process is complete for at least two reasons. First, it is desirable to maximize wafer throughput and not spend more time than necessary cleaning the chamber. Second, the cleaning process may etch chamber components after removing the residue, which may reduce the operating lifetime of these components. Optical endpoint detection methods have been used to determine the endpoint of in situ plasma clean processes. An optical endpoint detection system may use a photo detector and optical spectrometer to measure the wavelength of the light emitted by the in situ glow discharge. The glow discharge may emit light of one wavelength while etching the residue, and emit a different wavelength of light after the residue has been cleaned. The cleaning process endpoint may be detected when a predetermined change of wavelength has occurred.
This type of optical endpoint detection is not practical for remote plasma cleaning systems because there is no in situ glow discharge, thus one cannot measure its wavelength. Simply timing the remote plasma cleaning process to determine its endpoint may not be suitable because the time necessary to perform a remote plasma clean may vary with the number of layers, composition of layers, total layer thickness, changes in the exhaust or purge gas flow, and other factors. A change in any one factor may change the time needed to clean the chamber and require a recalibration of the cleaning process time. Therefore, simply timing the length of a remote plasma cleaning process to determine its endpoint may not sufficient to determine the endpoint of the cleaning process.
From the above, it can be seen that it is desirable to have an efficient and thorough remote plasma cleaning process. It is also desirable to provide an endpoint detection method and apparatus to enhance utilization of the substrate processing equipment and to reduce the overetching of chamber components. The endpoint detection method should provide a reliable indication of the end of the cleaning process under a variety of cleaning conditions.